iov_iter: Convert copy_from_user_iter() to masked user access

iov_iter: Convert copy_from_user_iter() to masked user access

copy_from_user_iter() lacks a speculation barrier, which will degrade
performance on some architecture like x86, which would be unfortunate as
copy_from_user_iter() is a critical hotpath function.

Convert copy_from_user_iter() to using masked user access on architecture
that support it. This allows to add the speculation barrier without
impacting performance.

This is similar to what was done for copy_from_user() in commit
0fc810ae3ae1 ("x86/uaccess: Avoid barrier_nospec() in 64-bit
copy_from_user()")

[ tglx: Massage change log ]

Signed-off-by: Christophe Leroy <christophe.leroy@csgroup.eu>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Link: https://patch.msgid.link/58e4b07d469ca68a2b9477fe2c1ccc8a44cef131.1763396724.git.christophe.leroy@csgroup.eu