mm: remove VM_EXEC requirement for THP eligibility

Commit e6be37b ("mm/huge_memory.c: add missing read-only THP checking
in transparent_hugepage_enabled()") introduced the VM_EXEC requirement,
which is not strictly needed.

lld's default --rosegment option and GNU ld's -z separate-code option
(default on Linux/x86 since binutils 2.31) create a read-only PT_LOAD
segment without the PF_X flag, which should be eligible for THP.

Certain architectures support medium and large code models, where .lrodata
may be placed in a separate read-only PT_LOAD segment, which should be
eligible for THP as well.

Link: https://lkml.kernel.org/r/20231220054123.1266001-1-maskray@google.com
Signed-off-by: Fangrui Song <maskray@google.com>
Acked-by: Yang Shi <shy828301@gmail.com>
Cc: Miaohe Lin <linmiaohe@huawei.com>
Cc: Song Liu <songliubraving@fb.com>
Cc: Matthew Wilcox <willy@infradead.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

Author: MaskRay

include/linux/huge_mm.h

@@ -206,7 +206,6 @@ static inline bool file_thp_enabled(struct vm_area_struct *vma)
 	inode = vma->vm_file->f_inode;
 
 	return (IS_ENABLED(CONFIG_READ_ONLY_THP_FOR_FS)) &&
-	       (vma->vm_flags & VM_EXEC) &&
 	       !inode_is_open_for_write(inode) && S_ISREG(inode->i_mode);
 }