fs_files.c: make sure that fs_getfilep is not interrupted when holding mutex

[raiden00pl]

Summary

• fs_files.c: make sure that fs_getfilep is not interrupted when holding mutex
  this fixes the issue Low priority tasks interrupt high priority threads #6012

Impact

fix #6012

Testing

b-g431b-esc1

Not tested enough for SMP targets (only some basic tests on qemu sabre-6quad/smp configuration).
I would be grateful if someone could help me verify whether this change doesn't break the performance of SMP in accessing file system or has other unwanted effects.

[zouboan]

Is there any problem with using spin_lock_irqsave and spin_unlock_irqrestore?

[raiden00pl]

The logic within spin_lock_irqsave() should not suspend (according to https://cwiki.apache.org/confluence/pages/viewpage.action?pageId=139629502).
In this case nxmutex_lock() may block an executing thread so xxx_critical_section() looks like the only way.

[zouboan]

got it,thank you!