ntfs: bound the attribute-list entry in ntfs_read_inode_mount()

In the Linux kernel, the following vulnerability has been resolved:

ntfs: bound the attribute-list entry in ntfs_read_inode_mount()

The $MFT attribute-list walk in ntfs_read_inode_mount() validates each
entry only with "(u8 *)al_entry + 6 > al_end" and
"(u8 *)al_entry + le16_to_cpu(al_entry->length) > al_end", but then reads
al_entry->lowest_vcn (an __le64 at offset 8) and al_entry->mft_reference
(offset 16) -- fields beyond the 6 bytes proven in range. al_entry->length
is attacker-controlled and only required non-zero, so a short entry (e.g.
length 8) placed at the tail passes both checks while the lowest_vcn /
mft_reference reads fall past al_end.

al_end is ni->attr_list + attr_list_size (the on-disk size); the buffer is
kvzalloc(round_up(attr_list_size, SECTOR_SIZE)), so the sector rounding
usually absorbs the over-read -- but when attr_list_size is a multiple of
SECTOR_SIZE there is no slack and a crafted $MFT attribute list produces an
out-of-bounds read at mount time.

Validate the entry with ntfs_attr_list_entry_is_valid() (added in patch
1/3) before dereferencing it, matching the bound the other attribute-list
walks now use. The validator already requires the length to cover the fixed
header, which makes the separate "!al_entry->length" check redundant, so
drop it too.