vfile: save memory by removing TOCTTOU protections

[hugelgupf]

On low memory systems, vfile contributes one of many copies of an OS to be kexec'd in u-root. To enable only-one-copy kexec, drop TOCTTOU protections.

[codecov]

Codecov Report

Attention: 1 lines in your changes are missing coverage. Please review.

Comparison is base (c5c7116) 77.31% compared to head (86e0890) 77.30%.

Files	Patch %	Lines
pkg/vfile/vfile.go	90.90%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #2880      +/-   ##
==========================================
- Coverage   77.31%   77.30%   -0.01%     
==========================================
  Files         429      429              
  Lines       43094    43088       -6     
==========================================
- Hits        33316    33308       -8     
- Misses       9778     9780       +2     

Flag	Coverage Δ
.-amd64	67.87% <ø> (ø)
cmds/...-amd64	71.35% <ø> (-0.06%)	⬇️
integration/generic-tests/...-amd64	18.21% <ø> (ø)
integration/generic-tests/...-arm	0.00% <ø> (ø)
integration/generic-tests/...-arm64	21.80% <ø> (ø)
integration/gotests/...-amd64	74.14% <90.90%> (+<0.01%)	⬆️
integration/gotests/...-arm	75.37% <90.90%> (+<0.01%)	⬆️
integration/gotests/...-arm64	75.09% <ø> (-0.01%)	⬇️
pkg/...-amd64	76.12% <90.90%> (-0.01%)	⬇️

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[chrisfenner]

LGTMing with the following assumptions in case I'm not seeing this right:

• TOCTTOU protection was best effort/aspirational for uroot kexec and wasn't a firm requirement in the first place
• Attempting TOCTTOU proof kexec requires loading the entire OS into memory and verifying it from there, which is not practical
• uroot users can protect themselves from threats that would exploit the TOCTTOU gap in some other way; or there is some future work-item to track creating a version of uroot that can provide this (perhaps at the cost of higher memory usage)